In the quest for low emissions, burners or combustors have been developed based, inter alia, on fundamental research concerning swirling flows, including the phenomenon of vortex breakdown, and on stability of premixed flames. A lean premixing technique can provide NO.sub.X emissions which are below 25 ppm when natural gas is used as a fuel and about 42 ppm when No. 2 oil is used as the fuel. In these burners, frequently a mixture of combustion air and fuel is injected into the combustor by means of a plurality of swirlers.
A free vortex is one of the simplest irrotational motions of an incompressible perfect fluid. Vorticity attaches to the fluid and is concentrated on the central axis. Such ideal vortices, however, cannot exist for a variety of reasons including, inter alia, the infinite energy required.
Whirls in the form of dust devils, water spouts and tornadoes occur in nature as the result of the simultaneous occurrence of an ambient vorticity produced for example in the ground boundary layer of a wind or wind shear or from the earth's rotation and a concentrating mechanism such as air rising in a buoyant column. The counterparts of these natural phenomenon, namely fire devils, fire whirls and fire tornadoes have attracted considerable interest in studies of fire which have been conducted over the last several years.
Laboratory studies on fire whirls were conducted by Byram and Martin (Fire Whirlwinds In The Laboratory, Fire Control Notes, January 1962, U.S. Forest Service) by positioning a stationary wall around a flame with the wall being interrupted by two vertical slots arranged so as to admit air in a tangential direction. Emmons and Ying, The Fire Whirl, llth Symposium (Int'l) on Combustion, The Combustion Institute, Pittsburgh, Pa. p. 475 (1967), describes a similar laboratory apparatus except that a rotating screen is used in place of the simpler slotted device. In both instances, cylindrical symmetry was observed. The fuel container and a tall cylindrical screen are arranged so as to be concentric. When the fuel is ignited and begins to burn, the screen is rotated and this in turn causes a rotation of the air. Since the angular momentum must be conserved, the rotational speed of the air near the fuel container is many times faster than that induced by the rotating screen or tangential air injection and as a result, the rotational air speed around the fuel is the air speed of the screen times the radius of the circular screen divided by the radius of the fuel container or nozzle. A swirling elongated flame is produced.
The present invention is based on the discovery that by introducing asymmetry to a fire whirl type configuration, a variety of advantageous effects can be achieved. Nitrogen oxide emissions in all types of combustors such as fuel burners, furnaces, automobile and gas turbine engine can be reduced. Fires such as oil well fires can be extinguished.